More than one reissue application has been filed for the reissue of U.S. Pat. No. 6,553,018. The reissue applications are: the present application, and application Ser. No. 12/178,013, filed on Jul. 23, 2008, which is a continuation of the present application.
The present invention relates to a communication terminal apparatus by means of CDMA (Code Division Multiple Access) utilizing spread spectrum system, and especially to a method of and an apparatus for controlling transmission power in one communication terminal apparatus in case of conducting multi-code transmission in which a plurality of spreading code channels are allocated to the communication terminal apparatus, and thereby, transmission capacity is enlarged.
In a mobile communication system having a base station and a plurality of mobile communication terminals, as a connection method in which the number of terminals capable of being accommodated in the system is increased, and which is capable of flexibly corresponding to a change of a transmission speed, CDMA in which a spread spectrum system is applied has been watched.
In case of mobile communication by means of the CDMA, especially in case of using Direct Sequence (DS) as a method of spread spectrum, if transmission power from the mobile communication terminals is the same as each other, since a reception electric field at the base station is generally in inverse proportion to a square of a distance between the base station and the mobile communication terminals, a weak radio wave from the terminal far from the base station is strongly interfered with a strong radio wave from the terminal near the base station, and a radio wave from the remote terminal cannot be normally received at the base station. Accordingly, it is necessary to control the transmission power for each terminal so that strength of received radio waves from each terminal is almost the same as each other at a position of the base station.
Conventionally, in the mobile communication system by means of the CDMA, it is usual that one code channel is allocated to one terminal that is used by a user. Here, the code channel is a communication channel that is specified by a spreading code (pseudorandom noise code) being used for spreading.
FIG. 7 is a block diagram simply showing a conventional mobile terminal 101 in accordance with a CDMA method from an aspect of transmission power control, and here, it is assumed that data transmission is conducted from the mobile terminal 101 to a base station 102 using only one code channel. The data to be transmitted is supplied to the mobile terminal 101 from a signal source 104 connected to the mobile terminal 101.
The data to be transmitted is often a voice signal, and is sometimes a high speed multi-media data that is output from a computer. In any event, it is assumed that the signal source 104 is for outputting a data stream at a bit rate of R bits/second.
Provided in the mobile terminal 101 are a receiver 112 connected to a reception antenna 111, a transmitter 114 connected to a transmission antenna 113, a spreading circuit 115 to which a data stream from the signal source 104 is input, a D/A (Digital/Analog) converter 116 for converting a digital signal output from the spreading circuit 115 into an analog signal, a modulator 117 for applying orthogonal modulation to a carrier wave based on an output from the D/A converter 116, a variable gain circuit 118 inserted between an output of the modulator 117 and an input of the transmitter 114. An oscillation circuit 119 for generating a high frequency signal that is a carrier signal is connected to the modulator 117.
After applying error correction coding and a process such as interleave and encrypt to a data stream from the signal source 104, the spreading circuit 115 spreads using a spreading code corresponding to an allocated code channel, and outputs a base band signal. Here, the spreading circuit 115 is constructed as a digital signal processing circuit, and spreads the data stream from the signal source 104 to generate a signal and outputs a multilevel digital signal as a base band signal, which represents an instant value of this signal every moment. Also, in the modulator 117, orthogonal modulation is conducted by means of four phases PSK (phase shift modulation) (QPSK (Quadrature Phase Shift Keying)), and accordingly, an in-phase component I and an orthogonal component Q of the base band signal are output from the spreading circuit 115 as a multilevel digital signal, respectively, and the D/A converter 116 independently converts the in-phase component I and the orthogonal component Q into analog signals, respectively, and the modulator 117 receives these in-phase component I and orthogonal component Q and conducts modulation.
Here, an arrangement of the spreading circuit 115 will be explained using FIG. 8. This spreading circuit 115 is for applying direct sequence (DS) system as spread spectrum system to the input data stream. In FIG. 8, values in parentheses show typical examples of a data speed, a chip rate and so forth.
A data stream of a data speed 128 kbps (bps is the number of bits per second) for example is input from the signal source, and a serial/parallel conversion circuit 121 with one input and two outputs (1→2) is provided for dividing this input data stream into two series of data streams with a data speed (64 kbps in this example) that is a half compared with the input data stream. One data stream from the serial/parallel conversion circuit 121 corresponds to the in-phase component I in the orthogonal modulation, and the other data stream corresponds to the orthogonal component Q. Disposed are a PN code generator 122 for generating a pseudorandom noise code (PN code) as a spreading code for the in-phase component I, and a PN code generator 123 for generating a pseudorandom noise code (PN code) as a spreading code for the in-phase component Q. The data stream and spreading code on a side of the in-phase component I are input to an adder 124, and thereby, the data stream corresponding to the in-phase component I is spreaded. In the same manner, the data stream and spreading code on a side of the orthogonal component Q are input to an adder 125, and thereby, the data stream corresponding to the orthogonal component Q is spreaded. The adders 124 and 125 are for calculating exclusive OR between the input data stream and the spreading code. A chip rate of a signal after spreading which is output from each of the adders 124 and 125 is 4.096 Mcps (cps is the number of chips per second), for example. The signals after spreading from the adders 124 and 125 are input to FIR (finite impulse response) filters 126 and 127 that function as a low pass filter, respectively, and thereby, a multilevel digital signal (8 bits value signal, for example) is output every moment, which represents an instant value of a base band signal of the in-phase component I and the orthogonal component Q.
In this manner, spread spectrum is applied to the data stream, and a transmission signal with a predetermined frequency band can be obtained from an output of the modulator 117. A level adjustment for this transmission signal is conducted by the variable gain circuit 118, and thereafter, this transmission signal is transmitted from the transmitter 114. The variable gain circuit 118 is constructed of an amplifier capable of varying a gain or an attenuator capable of varying an amount of attenuation. As mentioned below, a gain (or an amount of attenuation) in the variable gain circuit 118 is controlled by a TPC (Total Power Control) signal from the receiver 112 1 dB by 1 dB, for example.
Now, it is assumed that a bit rate of the data stream from the signal source 104 is R [bits/second], and a band width of the transmitted signal is W [Hz],G=W/R  (1)is called a spreading gain.
After receiving such a transmission signal from the mobile terminal 101, the base station 102 applies de-spreading, decoding, de-interleave and error correction to this signal. It is assumed that signal power per bit necessary for fully receiving this signal in the base station 102 is Eb, noise power per Hertz is N0, and a ratio of these is Eb/N0. Here, to fully receive the signal means that a bit error rate in a data stream output after the error correction is satisfied with a predetermined level. Then, a carrier/noise ratio (C/N) that is needed in the base station 102 is:                                                                         C                /                N                            =                                                (                                      R                    ·                                          E                      b                                                        )                                /                                  (                                      W                    ·                                          N                      0                                                        )                                                                                                        =                                                (                                      1                    /                    C                                    )                                ·                                  (                                                            E                      b                                        /                                          N                      0                                                        )                                                                                        (        2        )            
From this, a signal level that is needed in the base station 102 is represented as follows:C=N·(1/G)·(Eb/N0)  (3)
Then, the base station 102 transmits to each mobile terminal 101 a command for controlling transmission power so as to always make a reception level of the signal to be C. Particularly, in case that a signal level of a code channel received from a certain mobile terminal 101 is smaller than the value C, the base station 102 transmits a command for making transmission power of the mobile terminal 101 increase by a constant (1 dB, for example), and in case that the signal level is larger conversely, the base station 102 transmits a command for making the transmission power decrease to the mobile terminal. This command is called a TPC (Total Power Control) signal. This signal can be a command that makes the transmission power increase if its value is “1” for example, and that makes the transmission power decrease if its value is “0”.
The mobile terminal 101 receives the TPC signal at the receiver 112. The received TPC signal is output from the receiver 112 to the variable gain circuit 118, and the variable gain circuit 118 increases or decreases a gain by a constant (1 dB, for example) in accordance with the TCP signal. Thereby, the transmission signal level is adjusted to a level required by the base station 102. A method of controlling the transmission power of the mobile terminal in this manner is called closed loop control. This control method is generally used in as IS-95CDMA system and so forth, which is a mobile communication system in the United States.
By the way, in recent years, even in a mobile communication field, multimedia of a transmission data has been developed, and only low speed data communication using only voice is unsatisfying and a higher speed transmission method is needed, such as connection to an internet and image communication. As one method for meeting these needs, multi-code transmission is being watched.
Different from conventional transmission, the multi-code transmission is for increasing a transmission speed by allocating a plurality of code channels (two channels, for example) to one terminal. If the number of codes is N (where n≧2), in case that a bit rate per code is R0, a collective transmission rate RT is as follows:RT=N·R0  (4)Namely, compared with a case in which a single code channel is used, it is possible to make the transmission rate to be N times.
However, in case of conducting the multi-code transmission, it is required to finely control the transmission power every code channel due to a reason as mentioned below. The present invention deals with how to control the transmission signal power of the terminal in case of conducting the multi-code transmission.
As main application of the multi-code transmission, a case can be raised, in which a voice signal and a data signal are concurrently transmitted. Particularly, there is a case in which by using two code channels, one of the code channels is allocated to transmission of a voice signal such as a conversation and the other is allocated to transmission of a data signal for a file exchange between computers. In considering such a situation, allowable error rates are different from each other between the voice signal and the data signal, and while a bit error rate of about 10−3 is allowed for the voice signal, a bit error rate equal or less than 10−6 is sometimes necessary for the data signal. On the other hand, in order to improve a capacity of an entire mobile communication system in case of using spread spectrum system, it is important to reduce transmission power as a whole. Furthermore, it is considered that in a certain area of the mobile communication system, in a moment, while there are many mobile terminals that is conducting voice communication, there are to many mobile terminals that is conducting data communication so much. If considering the above, in the mobile terminal for conducting the multi-code transmission, without controlling the transmission power of code channels of both the voice signal and data signal so as to make both the transmission power of the code channels to be the same as each other based on a bit error rate required for the data signal, it becomes to be possible to make a capacity of the entire mobile communication system increase while meeting bit error rate required for the respective signals by making the transmission power of the code channel of the voice signal relatively small and making the transmission power of the code channel of the data communication relatively large, and also, it becomes to be possible to extend a time during which conversation by telephone can be made in accordance with a battery capacity of the mobile terminal.
As above, by raising the case, as an example, in which the voice signal and the data signal are allocated to the respective code channels, the reason why the transmission power control should be conducted for each code channel in case of conducting the multi-code transmission was explained. However, a kind of the signal to be transmitted is not necessarily limited to the voice signal such as conversation and the data signal for the file exchange. For example, there is also a case in which a dynamic image data, a static image data and so forth are transmitted. Also, even in the voice signal, there are a case such as conversation in which relatively low voice quality is allowed, and also, a case such as music in which relatively high quality is required. Even in data communication between computers, bit error rates required for a layer of the transmission by means of the CDMA are different from each other in accordance with an upward protocol. Therefore, it is required to determine a bit error rate and so forth in accordance with a kind and characteristic of a signal (data) to be transmitted, and to precisely conduct transmission power control in accordance with the determined ratio.
It can be also considered that a transfer route of a data to be transmitted is changed for each code channel. In considering that a band width of a signal after spreading is determined by a chip rate, since, if chip rates are the same, a spreading gain is improved as a data rate becomes to be lower, it is possible to make transmission power small by the improvement. From this point also, it is required to precisely conduct transmission power control for each code channel.
Now, out of the methods of conducting the transmission power control for each code channel in this manner, a method can most easily be imagined, in which a plurality of circuits from the signal source 104 to variable gain circuit 118 in the circuit shown in FIG. 7 are prepared in accordance with the number of code channels to be used, and outputs from the plurality of variable gain circuits are added to each other in an analog manner by a high frequency signal adder (wave coupler), and a signal after the addition is input to the transmitter. FIG. 9 is a block diagram showing an arrangement of a mobile terminal for controlling transmission power by means of the method mentioned here, in which N is equal to two, that is, the number of the code channels to be used in the terminal is two. In this mobile terminal 121, two series of circuits from the signal source 104 to the variable gain circuit 118 in the mobile terminal 101 shown in FIG. 7 are simply mounted. In FIG. 9, by adding one of letters A and B to a symbol, it is clearly shown that each component belongs to which series. In other words, a circuit corresponding to a code channel A is a circuit from the signal source 104A to the variable gain circuit 118A, and a circuit corresponding to a code channel B is a circuit from the signal source 104B to the variable gain circuit 118B. In addition, spread codes used in the spreading circuits 115A and 115B are spread codes of the code channels A and B, respectively, and therefore, the spreading circuits 115A and 115B use spread codes different from each other. Also, the oscillation circuit 119 for generating a carrier signal is provided for the modulators 117A and 117B in common. The outputs from both the variable gain circuits 118A and 118B are added to each other in an analog manner by the signal adder (wave coupler) 122, and an output from the signal adder 122 is input to the transmitter 114. As a result, a signal generated by adding the transmission signals of the code channels A and B to each other is transmitted to the base station 102.
The base station 102 considers the code channels A and B as individual channels, respectively, and transmits a TPCA signal and a TPCB signal to the mobile terminal 121, which are TPC signals for each code channel. The mobile terminal 121 receives a signal from the base station 101, and controls the variable gain circuits 118A and 118B, respectively, using the received TPCA signal and TPCB signal. Thereby, closed loop power control for each code channel is conducted.
However, in case that the control of the transmission power for each code channel is conducted using the circuit shown in FIG. 9 in conducting the multi-code transmission, there is a task that this circuit is simply constructed by mounting a plurality of circuits from the signal source to the variable gain circuit just before the transmitter, which corresponds to the number of the multi-code, and that a circuit scale thereof is not different so much from that in the case of using individual mobile terminals for each code channel. Especially, since high frequency circuits such as modulators and D/A converters are provided in accordance with the number of the code channels, electric power consumption becomes to be larger than that in a usual terminal. Especially, to provide a plurality of D/A converters directly results in the increase of electric power consumption. Ultimately, in the arrangement shown in FIG. 9, the sense that the terminal appropriate for the multi-code transmission is arranged cannot be accomplished. Furthermore, since there are a plurality of code channels in the case of the multi-code transmission, the control of the transmission power is complicated, and in the arrangement shown in FIG. 9, there is also a task that this control is not rationalized.